start with two quarts.  You will probably need more though.  Three coats is really not enough if you are sanding between coats.  The first three coats should be thinned as per instructions.  so you really arent building up all that much of a base.   You could also try the high build no sanding between coats products.  You have to sand out the runs and nibs, which you will have, but it will build up faster.  Five coats minimum for any areas under duress.  Seats, rails, etc.  Then after you use her for a season, you will need to add more.  It is an endless process but well worth it.

Thanks, Bill. I've already got two quarts, so at least I have enough to get started once we get some warm days.

Jeff  

   I finished the interior of my dory bright and used almost 2 quarts of gloss spar varnish on the hull and on the thwarts and all the sailing rig pieces (d-board, rudder, spars, mast partner).  Intended to top that off with at least one coat of satin varnish to make it easier on the eyes (less glare) but the desire to get out sailing took over and I spent one whole season slip-sliding on the glossy deck.  THe next spring I put 2 more coats of spar varnish and then topped it with satin.  Huge improvement in looks and not slipping ... a safety plus. I left the thwarts, spars etc. glossy.  

I treat my boat pretty gently and yet the outside of the hull does get scratched up on beach landings, dock contact, etc.  I would not be a fan of leaving the outside bright.   The lapstrake looks great painted in a light color and after a season of scratches, you don't have to repaint the whole thing, you can do a touchup on spots.